1. Field of the Invention
The present invention relates generally to a data processing device, particularly to a data processing device that erases inter-symbol interference (ISI) of the transmitted data through a timing recovery circuit and extracts a correct sampling time.
2. Description of the Related Art
The growing popularity of data communication and the trend towards user's demand for transmitting audio and video at a very high baud rate have resulted in a great workload for communication network. A promising method is to transmit digital data by using a narrow-band telephone line; however, in existing user-subscribed loop the transmission will be restricted due to inter-symbol interference (thereafter, ISI), if data transmission is undertaken at an extremely high baud rate.
An interface circuit of a user-subscribed loop generally comprises a transmitter and a receiver. In an integrated service digital network (thereafter, ISDN) system, the speed of a U-interfaced transceiver transmits and receives digital data is set at 160 Kb/s, as shown in FIG. 1B. In the transceiver a data frame 310 of 240 bit or 120 baud (including 9 frame sync-words) is formed and then encoded into a pseudo-random bit stream by a scrambler, the bit stream is further converted into a 2B1Q (two binary, one quaternary) code through a 2B1Q encoder according to ANSI Working Group T1E1 standard, and combinations of binary bit 00, 01, 10, 11 are mapped to 4 symbol levels, such as -3, -1, +1, +3. Next, before the signal is delivered to a remote place via a user-subscribed loop, the transmitter will convert the symbol levels into analog pulses with a filter. In the aforementioned frame sync-word SW={sw0, sw1, sw2, sw3, sw4, sw5, sw6, sw7, sw8}={+3, +3, -3, -3, -3, +3, -3, +3, +3}.
The signal outputted to the U-interfaced transceiver is sampled at 80 Kbaud per second, the sampled sample will be processed by the transceiver to extract original digital data, and finally be determined to be 0 or 1 by the DSP, wherein a typical impulse response of transmission channel is shown as in FIG. 1A, the typical impulse response, after being sampled by the transceiver, generates a plurality of h(n) discrete channel response, such as h.sub.-2, h.sub.-1, h.sub.0, h.sub.1, h.sub.2, . . . h.sub.N, wherein the main pulse is defined as h.sub.0, and the peak of the pulse is generally called a main "cursor." Moreover, the channel responses h.sub.-1 .about.h.sub.-2 are called pre-cursors, and the gradually decreasing channel responses h.sub.1 .about.h.sub.N with a long tail 10 are called post-cursors. Both of them often create inter-symbol interference to adjacent main pulses. The sampling timing t.sub.0 is normally chosen at a time when the peak of the pulse meets a determined value h.sub.0, that is, the time when the maximal h.sub.0 will be obtained.
Since pulses will lose their fidelity in the transmission process and become fuzzier, thus are difficult for a receiver to detect. The two main causes leading to losing pulse fidelity are, firstly due to the inter-coupling of pulses across the hybrid circuit, and this is a common problem in dual-line system signal transmission commonly known as an echo and is erased with an echo-erasing device in the receiver.
Another cause of losing pulse fidelity is due to ISI, as shown in FIG. 1A. Sometimes, the tail 10 of the impulse response of a fidelity-losing pulse can extend to tens of subsequent main pulses, therefore, ISI occurs when a signal is transmitted at the time of sampling, that is, the signal the receiver receives is more than just the current main cursor component, the sampled result further includes the interference component caused by a pre-cursor and the interference component caused by a tailed post-cursor.
As mentioned above, the sampling timing t.sub.0 is normally arranged at the time when the peak of the pulse meets a determined value h.sub.0, that is, the time to obtain the maximal h.sub.0. But when baud-rate sampling position is not on the peak of the pulse, a timing error will occur. As a result, the probability for a DSP to make wrong decisions on data gets higher; therefore, a timing recovery device is required to correct baud-rate sampling positions. At the phase when the receiver adjusts its coefficient, that is, in the training state, the signal can be delivered to a sync-word matching filter to obtain the value of a timing error and to estimate the timing error from the obtained impulse response. However, the impulse response has a tailed post-cursor that makes the estimate difficult to be precise and is therefore not applicable in a steady state (after the adjustment coefficient of the receiver has been optimized).